Internal combustion engines are controlled during idle by an electronic throttle control valve or blade that is relatively large and incapable of making small adjustments to the intake airflow. As such, throttle valves do not adequately control idle speed. To address this limitation, a separate idle air control valve has been used to define smaller adjustments to the intake airflow and therefore the engine idle speed.
Other control systems use spark timing to adjust engine idle speed. By advancing or retarding spark timing the torque output and engine speed are adjusted. The spark timing is usually adjusted to achieve the mean best torque (MBT). As such, advancing and retarding of the spark from the MBT reduces engine performance and increases fuel consumption.